Choi Eun-seol (Shin Se-kyung) arrives home to find her parents murdered. When the murderer is distracted, she escapes but is hit by a car. Eun-seol’s parents are later found with a barcode carved into their skin, the work of the “Barcode” serial killer. The lead detective on the case, Oh Jae-pyo (Jung In-gi) realizes that the now-comatose daughter Eun-seol is the sole surviving witness.
On that same night, Choi Moo-gak (Park Yoo-chun) is at the hospital visiting his younger sister (Kim So-hyun), also named Choi Eun-seol, who’s being treated for mild injuries after a bus accident. But when he returns to her bed, he finds her dead with her throat slit.
Six months later, Eun-seol comes out of her coma. But she has no memories at all of her life before waking up at the hospital, and her left eye has turned green with no scientific explanation. She has also gained the unique ability to “see” smells as visible colors and shapes, and can even trace where people have been because their lingering scents are like a trail. To protect her, Jae-pyo adopts her and invents a new life for her, telling her that he’s her real father and that her name is Oh Cho-rim. Three years and six months pass, and Cho-rim is a cheerful girl who’s adjusted to her ability. She dreams of a becoming a comedian, and works as a gofer for a small, struggling theater company called Frog Troupe.
On the other hand, Moo-gak had been so traumatized by his sister’s death that after two months of no sleep and constant pain, he’d fainted and was clinically dead for ten days. When he woke up, he’d lost the sense of smell and taste, and is unable to feel pain. And unlike his previous warm personality, Moo-gak is now stoic, emotionless, and speaks in a monotone. He leaves his job at the aquarium to become a police officer, vowing to reopen the case and catch the killer. But to do so, he must first get promoted to detective, and has to prove himself to the head of the homicide unit Detective Kang Hyuk (Lee Won-jong) in ten days or to never ask for a promotion again. Cho-rim also needs a skit partner for her upcoming audition in ten days, and when she offers to help him with his cases in exchange, they decide to team up. After a model (Park Han-byul) turns up dead, seemingly another victim of the Barcode serial killer, Moo-gak joins the investigation headed by Lieutenant Yeom Mi (Yoon Jin-seo), and he hones in on two possible suspects: the model’s boyfriend, chef Kwon Jae-hee (Namgoong Min) and her doctor (Song Jong-ho).